In motor vehicles automatic or automated transmissions are used more and more often. In such transmissions automated friction force locking clutches and in some cases automated interlocking clutches are used; the present invention concerns automated friction force locking clutches, also referred to as automated friction clutches.
To ensure comfortable and low-wear operation of an automated friction clutch, it is necessary to have as accurate as possible a knowledge of the relationship between a control parameter and a torque transmitted by the friction clutch. The control parameter can be, for example, a so-termed clutch pressure by which the friction clutch is actuated to close or open it. Typically, the relationship between a control parameter and the torque is stored in a control unit of the motor vehicle in the form of a characteristic torque curve as a function of the control parameter, for example as a function of the clutch pressure, as for example in a transmission control unit of the motor vehicle, and during operation the friction clutch is actuated on the basis of such a torque characteristic.
An essential parameter or reference point of a torque characteristic of an automated friction clutch is the so-termed touch-point. The touch-point is also referred to as the contact point or engagement point or feel-point of the friction clutch.
The touch-point of the torque characteristic of an automated friction clutch is understood to be a reference point at which, in a clutch actuated in the engagement or closing direction, friction elements of the friction clutch just come into contact so that the friction clutch just begins transmitting a minimal torque, and at which, when the clutch is actuated in the disengagement or opening direction, the friction elements of the friction clutch are just separated so that the frictional transmission of a torque by the friction clutch is just interrupted.
During the operation of the friction clutch, as a result of clutch wear the touch-point of the friction clutch can change. Moreover, in the mass production of friction clutches with the same structure, because of manufacturing tolerances the touch-points of individual friction clutches can differ from one another.
In order nevertheless to ensure the comfortable and low-wear operation of an automated friction clutch, from the prior art it is already known to determine the touch-point of a friction clutch adaptively. DE 10 2008 043 384 A1, for example, describes such a method for the adaptive determination of a current, actual touch-point of a friction clutch.
According to the prior art, the current touch-point determined adaptively in this way is used as the control-relevant touch-point. From the prior art it is also known to determine a control-relevant touch-point from an adaptively determined current touch-point by modifying the adaptively determined current touch-point with a constant offset value. This constant offset value is also referred to as the fine-adaptation value.